The Blue Ensign is a British ensign that may be used on vessels by certain authorised yacht clubs, Royal Research Ships and British merchant vessels whose master holds a commission in the Royal Naval Reserve or has otherwise been issued a warrant.

Defaced versions with a badge or other emblem are used more broadly; in the United Kingdom by authorised government or private bodies; and internationally by nations or organisations previously a part of the British Empire.

The ensign originated in the 17th century with the St George's Cross (the Flag of England) in the canton, and with a blue field.

In addition, prior to WWI, a number of ships (in particular, the RMS Lusitania and RMS Mauretania) were listed as Royal Navy Reserve Merchant Vessels, receiving an annual subsidy from the navy and were permitted to fly the blue ensign.

Current defaced Blue Ensigns (besides yacht clubs listed below) are: Royal Air Force marine vessels (such as seaplane tenders) flew a defaced blue ensign with an eagle and anchor.